U.S. patent number 7,816,269 [Application Number 11/714,223] was granted by the patent office on 2010-10-19 for plasma deposition apparatus and method for making polycrystalline silicon.
This patent grant is currently assigned to Silica Tech, LLC. Invention is credited to Mohd A. Aslami, DeLuca Charles, Dau Wu.
United States Patent |
7,816,269 |
Aslami , et al. |
October 19, 2010 |
Plasma deposition apparatus and method for making polycrystalline
silicon
Abstract
A plasma deposition apparatus for making polycrystalline silicon
including a chamber for depositing said polycrystalline silicon,
the chamber having an exhaust system for recovering un-deposited
gases; a support located within the deposition chamber for holding
a target substrate having a deposition surface, the deposition
surface defining a deposition zone; at least one induction coupled
plasma torch located within the deposition chamber and spaced apart
from the support, the at least one induction coupled plasma torch
producing a plasma flame that is substantially perpendicular to the
deposition surface, the plasma flame defining a reaction zone for
reacting at least one precursor gas source to produce the
polycrystalline silicon for depositing a layer of the
polycrystalline silicon the deposition surface.
Inventors: |
Aslami; Mohd A. (Sturbridge,
MA), Wu; Dau (Fallbrook, CA), Charles; DeLuca (South
Windsor, CT) |
Assignee: |
Silica Tech, LLC (White Plains,
NY)
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Family
ID: |
38446010 |
Appl.
No.: |
11/714,223 |
Filed: |
March 6, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080009126 A1 |
Jan 10, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60818966 |
Jul 7, 2006 |
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Current U.S.
Class: |
438/689;
422/186 |
Current CPC
Class: |
C23C
16/24 (20130101); H05H 1/30 (20130101); C23C
16/513 (20130101) |
Current International
Class: |
H01L
21/302 (20060101); H01L 21/461 (20060101) |
Field of
Search: |
;422/186 ;438/689 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 286 306 |
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Oct 1988 |
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EP |
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1 281 680 |
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Feb 2003 |
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EP |
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Other References
"Ultrafast deposition of microcrystalline Si by thermal plasma
chemical vapor deposition", Journal of Applied Physics, vol. 89,
No. 12, Jun. 15, 2001. cited by other.
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Primary Examiner: Toledo; Fernando L
Attorney, Agent or Firm: Patton Boggs LLP
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
No. 60/818,966, filed Jul. 7, 2006.
Claims
What is claimed:
1. A plasma deposition apparatus for making polycrystalline silicon
comprising: chamber means for depositing said polycrystalline
silicon; means for supporting a target substrate having a
deposition surface; induction coupled plasma torch means for
producing a plasma flame for reacting at least one reactant to
produce a reaction product and depositing said reaction product on
said target substrate, said plasma torch means located a fixed
distance from said substrate, wherein said means for supporting
moves said target substrate in a direction away from said induction
coupled plasma torch means to provide said fixed distance between
said target substrate and said induction coupled plasma torch
means; and at least one injection port disposed between said
induction coupled plasma torch and said deposition surface, said at
least one injection port connected to a precursor chemical source
for injecting said precursor chemical source into said induction
coupled plasma torch.
2. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said reaction product is selected from
the group consisting of silicon, intrinsic silicon, p-type doped
silicon, and n-type doped silicon.
3. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said at least one reactant is in the
form of a material selected from the group consisting of a gas,
vapor, aerosol, small particle, nanoparticles, or powder.
4. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said at least one reactant is hydrogen
(H.sub.2) and at least one compound selected from the group
consisting of trichlorosilane (SiHCl.sub.3), silicon tetrachloride
(SiCl.sub.4), dichlorosilane (SiH.sub.2Cl.sub.2), Silane
(SiH.sub.4), Disilane (Si.sub.2H.sub.6), Silicon tetrabromide
(SiBr.sub.4), and mixtures thereof.
5. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said plasma flame is produced from at
least one gas selected from the group consisting of helium gas,
argon gas, hydrogen gas, and mixtures thereof.
6. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said chamber means further includes: an
exhaust means located above said deposition surface for exhausting
at least one of un-deposited solids and un-reacted chemicals from
said chamber means.
7. The plasma deposition apparatus for making polycrystalline
silicon of claim 6 wherein said chamber means further includes: a
recycling means for recycling said at least one of said
un-deposited solids and unreacted chemicals exhausted from said
chamber means for re-use in said deposition apparatus.
8. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said induction coupled plasma means
further comprises: an induction coil that comprises a plurality of
windings having a diameter greater than that of said outer quartz
tube and spaced apart from each other by distance of about 2-10
mm.
9. The plasma deposition apparatus for making polycrystalline
silicon of claim 8 wherein said distance between said induction
coil and said target substrate is between about 30-55 mm.
10. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said induction coupled plasma torch and
said deposition surface are substantially perpendicular to each
other.
11. The plasma deposition apparatus for making polycrystalline
silicon of claim 10 wherein said induction coupled plasma torch is
positioned substantially vertically.
12. The plasma deposition apparatus for making polycrystalline
silicon of claim 1 wherein said deposition surface is rotated
during deposition of said reaction product.
13. A plasma deposition apparatus for making polycrystalline
silicon comprising: a chamber for depositing said polycrystalline
silicon, said chamber having an exhaust system for recovering at
least one of un-deposited solids and un-reacted chemicals; a
support located within said deposition chamber for holding a target
substrate having a deposition surface, said deposition surface
defining a deposition zone; at least one induction coupled plasma
torch located within said deposition chamber and spaced apart from
said support, said at least one induction coupled plasma torch
producing a plasma flame that is substantially perpendicular to
said deposition surface, said plasma flame defining a reaction zone
for reacting at least two reactants to produce said polycrystalline
silicon for depositing a layer of said polycrystalline silicon said
deposition surface; and at least one injection port disposed
between said at least one induction coupled plasma torch and said
deposition surface, said at least one injection port connected to a
precursor chemical source for injecting said precursor chemical
source into said at least one induction coupled plasma torch.
14. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said polycrystalline silicon is
selected from the group consisting of silicon, intrinsic silicon,
p-type doped silicon, and n-type doped silicon.
15. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said at least two reactants are
deposited in the form of a material selected from the group
consisting of a gas, vapor, aerosol, small particle, nanoparticles,
or powder.
16. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said at least two reactants are
produced by hydrogen (H.sub.2) and at least one gas selected from
trichlorosilane (SiHCl.sub.3), silicon tetrachloride (SiCl.sub.4),
dichlorosilane (SiH.sub.2Cl.sub.2), Silane (SiH.sub.4), Disilane
(Si.sub.2H.sub.6), Silicon tetrabromide (SiBr.sub.4), and mixtures
thereof.
17. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said plasma torch means is produced
from at least one gas selected from the group consisting of helium
gas, argon gas, hydrogen gas, and mixtures thereof.
18. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said chamber means further includes: an
exhaust system located above said deposition surface for exhausting
said at least one of un-deposited solids and un-reacted chemicals
from said chamber means.
19. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said chamber for depositing is made
from a material that shields RF energy and isolates said chamber
from the environment outside of said chamber.
20. The plasma deposition apparatus for making polycrystalline
silicon of claim 19 wherein said exhaust system further comprises:
exhaust ports for removing by-product gases and particles from said
chamber.
21. The plasma deposition apparatus for making polycrystalline
silicon of claim 18 wherein said exhaust system controls the
partial pressure in said chamber.
22. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 wherein said at least one induction coupled
plasma torches comprises: an outer quartz tube and an induction
coil comprising a plurality of windings having a diameter greater
than that of said outer quartz tube; an inner quartz tube; and a
chamber connecting said outer quartz tube and said inner quartz
tube, wherein said plasma gas source is connected to said chamber
to provide said plasma gas source between said outer quartz tube
and said inner quartz tube.
23. The plasma deposition apparatus for making polycrystalline
silicon of claim 22 wherein said outer quartz tube has a length of
about 180-400 mm.
24. The plasma deposition apparatus for making polycrystalline
silicon of claim 22 wherein said outer quartz tube has a diameter
of about 50-90 mm.
25. The plasma deposition apparatus for making polycrystalline
silicon of claim 22 wherein said inner quartz tube has a length of
about 120-180 mm.
26. The plasma deposition apparatus for making polycrystalline
silicon of claim 22 wherein said inner quartz tube has a diameter
of about 50-70 mm.
27. The plasma deposition apparatus for making polycrystalline
silicon of claim 22 wherein said windings are spaced apart from
each other by distance of about 2-10 mm.
28. The plasma deposition apparatus for making polycrystalline
silicon of claim 27 wherein said distance between said induction
coil and said target substrate is between about 30-55 mm.
29. The plasma deposition apparatus for making polycrystalline
silicon of claim 22 further comprising a high frequency generator
connected to said induction coil.
30. The plasma deposition apparatus for making polycrystalline
silicon of claim 13 further comprising: recycling said recovered
said un-deposited solids to be processed into an ingot.
Description
FIELD OF THE INVENTION
The present invention relates to an apparatus and process for
making polycrystalline silicon.
Problem
As oil prices have continued to increase and other energy sources
remain limited, there is increasing pressure on global warming from
the emissions of burning fossil fuel. There is a need to find and
use alternative energy sources, such as solar energy because it is
free and does not generate carbon dioxide gas. To that end, many
nations are increasing their investment in safe and reliable
long-term sources of power, particularly "green" or "clean" energy
sources. Nonetheless, while the solar cell, also known as a
photovoltaic cell or modules, has been developed for many years, it
had very limited usage because the cost of manufacturing these
cells or modules is still high, making it difficult to compete with
energy generated by fossil fuel.
Presently, the single crystal silicon solar cell has the best
energy conversion efficiency, but it also has high manufacturing
cost associated with it. Alternatively, polycrystalline silicon
while it does not have the same high efficiency of a single crystal
cell, it is much cheaper to produce. Therefore, it has the
potential for low cost photovoltaic power generation. One known
method for making a single crystal ingot is to use a floating zone
method to reprocess a polycrystalline silicon rod. Another known
method is the Czochralski method that uses a seed crystal to pull a
melted silicon from a melting crucible filled with polycrystalline
silicon nuggets.
In addition, some prior art processes of making polysilicon use
chlorosilanes that are dissociated by resistance-heated filaments
to produce silicon, which is then deposited inside a bell-jar
reactor. It is commonly known to make a semiconductor grade silicon
with trichlorosilane and then later recycle these chlorosilanes.
Also, there have been many attempts using different raw materials
to make polysilicon followed by re-processing these un-reacted
chemicals. Nevertheless, these previous attempts do not have a high
deposition rates.
Another attempt is to use a high pressure plasma with chlorosilane
to make polycrystalline silicon, and then recycle the un-reacted
chemicals. In this attempt, the deposition takes place on the
inside wall of a substrate to form a sheet type silicon that will
eventually be separated from the substrate, thus requiring
additional process steps.
In addition, a commonly known process involves making a solar cell
by (i) manufacturing polycrystalline silicon, (ii) making either a
single crystal or a polycrystalline ingot or block, (iii) making
wafers from the ingot or block (iv) and then making a cell, that
includes the step of p-type and n-type doping via a costly
diffusion process. The p-type and n-type dopants form the p-n
junction of the semiconductor material. This step is normally done
in extremely slow diffusion furnaces after the thin-film layer has
already been deposited, thus further slowing down the overall
process of efficiently producing solar cells.
In addition, prior art methods have the deposition surface parallel
to the plasma flame stream, thus the collection efficiency is much
lower. The gaseous silicon hydrides are deposited using a
high-frequency plasma chemical vapor deposition process to deposit
silicon on a horizontal silicon core rod. Because of the
orientation of the deposition apparatus, much of the silicon
products are exhausted out of the apparatus.
Further known prior art methods for producing silicon create
internal strain within the silicon rod. An attempt to reduce the
internal stress follows the basic Siemens process and making the
silicon rod in a bell-jar, where the process steps are: heating a
silicon core material in a gaseous atmosphere including
trichlorosilane and hydrogen to deposit silicon on the silicon core
material to produce a polycrystalline silicon rod, heating the
polycrystal silicon rod by applying an electric current without
allowing the polycrystal silicon rod to contact with air so that
the surface temperature of the polycrystal silicon rod is higher
than the deposition reaction temperature of silicon and is
1,030.degree. C. or higher, and shutting off the electric current
after the heating by reducing the applied current as sharply as
possible, thereby attempting to reduce the internal strain rate of
the polycrystal silicon rod. As can be seen, this process involves
a plurality of additional steps.
In another attempt to produce a polycrystalline silicon metal from
a silicon halide plasma source, the silicon halide is split into
silicon and halide ions in an inductively coupled plasma and
silicon ions are then condensed to form molten silicon metal that
can be vacuum cast into polysilicon ingots. In addition, the laden
gases are fluorine and chlorine. Fluorine and hydrogen fluoride are
highly corrosive, thus they require special corrosion resistant
material for building the equipment and when handling these
chemicals special case must be taken.
Information relevant to attempts to address these problems can be
found in the U.S. Pat. No. 4,292,342 issued 29 Sep. 1981 to Sarma
et al.; U.S. Pat. No. 4,309,259 issued 5 Jan. 1982 to Sarma et al.;
U.S. Pat. No. 4,321,246 issued 23 Mar. 1982 to Sarma et al.; U.S.
Pat. No. 4,491,604 issued 1 Jan. 1985 to Lesk et al.; U.S. Pat. No.
4,590,024 issued 20 May 1986 to Lesk et al., U.S. Pat. No.
5,976,481 issued 2 Nov. 1999 to Kubota et al.; U.S. Pat. No.
6,503,563 issued 7 Jan. 2003 to Yatsurugi et al; and U.S. Pat. No.
6,926,876 issued 9 Aug. 2005 to Kelsey.
Solution
The above-described problems are solved and a technical advance
achieved by the present plasma deposition apparatus and method for
making polycrystalline silicon disclosed in this application. The
present plasma deposition apparatus includes a deposition chamber
that contains preferably a set or plurality of induction coupled
plasma torches. The induction coupled plasma torches are oriented
substantially perpendicular from a deposition surface of a target
substrate to produce a large deposition area on the target
substrate. By being substantially perpendicular to the deposition
surface, the polycrystalline silicon that is created in the
reaction zone near the end of the induction coupled plasma torches
flows directly towards the perpendicular substantially planar
deposition surface. In addition, the present plasma deposition
apparatus rotates the target substrate during deposition to produce
a uniform layer of polycrystalline silicon on the deposition
surface. Also, a support moves the target substrate away from the
induction coupled plasma torches during deposition to provide a
constant or fixed distance between the induction coupled plasma
torches and the deposition surface.
The present method for making polycrystalline silicon eliminates
the manufacturing of polysilicon as a separate process and also
eliminates the additional p-type and n-type doping processing steps
that occur subsequently. In addition, the present method for making
polycrystalline silicon can also use and reuse the same raw
materials, because the un-reacted or un-deposited chemicals can be
collected and recycled for reprocessing through the present
apparatus.
The present method for making polycrystalline silicon does not use
different types of material as a substrate, thus there is no
additional process for separation. The present method for making
polycrystalline silicon eliminates the additional processing steps
found in conventional deposition methods, which can cause extra
process loss. Further, the present method for making
polycrystalline silicon also minimizes the potential contamination
of the substrate.
The present method for making polycrystalline silicon has no
limitation on the size of the target area and the produced silicon
ingot can be removed to provide a continuous process. The present
method for making polycrystalline silicon also separates the
reaction zone from the deposition zone. By doing so, the process
temperature of the reaction zone can be thermodynamically optimized
for higher chemical reaction efficiency. Moreover, in the
deposition zone, optimal temperatures for better deposition
efficiency and product quality can be achieved. Since the
deposition surface of the silicon ingot is facing substantially
perpendicular to the plasma flame of the induction coupled plasma
torch, a larger collecting or deposition surface is available for
silicon deposition. The vertical deposition method as herein
disclosed has a higher deposition rate than that achieved in a
deposition of a curved surface, such as on a rod form.
The novel process uses at least one induction coupled plasma torch
aligned substantially perpendicular to the deposition surface
target to deposit silicon on the vertical axis of the target. Using
more than one induction coupled plasma torch will further increase
the deposition rate and will increase the deposition area of the
target, which can further reduce manufacturing costs of a solar
cell. It has a higher deposition rate, and it can be designed as a
continuous flow process such that it can dramatically lower the
manufacturing cost of a polycrystalline silicon. Its novel design
provides for a better separation of the reaction and collection
processes. Through its design, the present apparatus accomplishes
increased reaction temperatures, thus higher reaction conversions
while concurrently providing more optimal product collection
temperatures.
The novel apparatus and method for making polycrystalline silicon
produces doped or undoped silicon ingots in one step and at a very
high deposition rate, thus producing finished and semi-finished
silicon ingots from polysilicon raw materials economically with
much less capital investment than the standard polysilicon
manufacturing processes. In addition, a dopant, such as boron or
phosphorous and the like, can be deposited simultaneously to
generate a p-type or n-type ingot, thus eliminating a costly
diffusion process down stream of the cell manufacturing process.
The plasma deposition apparatus and method for making
polycrystalline silicon further provides better deposition control
and more uniform dopant distribution while eliminating the
diffusion step of conventional processes, thus yielding a higher
production rate of doped silicon.
Also, the plasma deposition apparatus and method for making
polycrystalline silicon can collect, separate, and recycle most of
the process gases and the un-deposited chemicals. These
un-deposited chemicals can then be further processed into either
single-crystal silicon or polycrystalline silicon.
The present methods for making polycrystalline silicon does not use
a bell-jar, and most likely will not experience the same stress
problems associated with that prior art process. This is because
the silicon is deposited on the end of the target substrate, and
thus it will have less temperature differential in the radial
direction that prior art processes and also because the target
substrate is rotated during deposition. In addition the present
novel process is a one-step deposition process that makes
polycrystalline silicon ingots, and it does not require a vacuum
casting step. It simplifies the manufacturing process, and will
lower the capital investment and operating cost of producing a
polycrystalline silicon.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a cutaway side view of the plasma deposition
apparatus for making polycrystalline silicon according to an
embodiment of the present invention;
FIG. 2 illustrates a top view of a plasma deposition apparatus
including several plasma deposition torches relative to a substrate
housed in a chamber according to another embodiment of the present
invention;
FIG. 3 illustrates a side view of the plasma deposition apparatus
including several plasma deposition torches relative to a substrate
housed in a deposition chamber of FIG. 2 according to another
embodiment of the present invention;
FIG. 4 illustrates a side view of a plasma deposition apparatus
including tilted induction coupled plasma torches relative to a
substrate housed in a deposition chamber according to another
embodiment of the present invention; and
FIG. 5 illustrates a flow diagram of a process for making
polycrystalline silicon according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an embodiment 100 of plasma deposition apparatus
including an induction coupled plasma torch 102 positioned below a
supported target substrate 104 held by support 103. The target
substrate 104 may be any desired size including those sizes
commonly known in the art of solar cells. In this embodiment, the
induction coupled plasma torch 102 is aimed upward for depositing a
reaction product on the deposition surface 106 of the target
substrate 104. In another embodiment, the induction coupled plasma
torch 102 may be aimed or oriented in another manner or direction
with respect to the target substrate 104. The induction coupled
plasma torch 102 consists of two concentric quartz tubes: an outer
quartz tube 108 and a shorter inner quartz tube 110, which are
shown to be attached to a stainless steel chamber 112.
Typically, the diameter and height or length of the outer quartz
tube 108 and the inner quartz tube 110 may be any size to fit the
desired application of the outer quartz tube 108 and inner quartz
tube 110. Preferably, the inner quartz tube 108 has a shorter
length than the outer quartz tube 108. Also, the outer quartz tube
108 preferably has a diameter in the range of from about 50
millimeters ("mm") to about 90 mm and a height in the range of from
180 mm to about 400 mm. More preferably, the diameter for the outer
quartz tube 108 is about 70 mm with a height or length of about 200
mm. Preferably, the inner quartz tube 110 has a diameter in the
range of from about 50 mm to about 70 mm and a height in the range
of from about 120 mm to about 180 mm. More preferably, the diameter
of the inner quartz tube 110 is about 60 mm with a height of about
150
The target substrate 104 may be an ingot or other form of
polycrystalline silicon substrate. In this embodiment, the
deposition surface 106 is substantially perpendicular to the
induction coupled plasma torch 102. Preferably, the support 103
rotates the target substrate 104 about its axis 107. In addition,
support 103 further moves the target substrate 104 away from the
induction coupled plasma torch 102 as the silicon layer is
deposited on the deposition surface 106 to keep the distance "L"
constant during the deposition process. Deposition of the silicon
takes place on the deposition surface 106 of the substrate and this
area of activity is noted generally as the deposition zone 105. The
distance between the uppermost portion of the coil 114 and the
target substrate 104, as designated by "L" is in the range of from
about 10 mm to about 40 mm depending on the deposition temperature.
Support 103 may be any type of mechanical support capable of
supporting, rotating, and/or moving the target substrate 104 during
a deposition process. Preferably, support 103 is resistant to high
temperatures and has sufficient mechanical strength to support the
deposition target 104 while moving the deposition target 104 up and
down and rotating the deposition target 104. In one embodiment, the
support is a rod-shaped rigid member that is connected to the
deposition target 104. Further it may be coupled to motors for
rotating the deposition target 104. In one embodiment, the support
103 is a quartz rod or high temperature corrosion resistant
stainless steel rod.
In addition to the deposition zone 105, the plasma deposition
apparatus 100 further includes a reaction zone 126 that is
separated a distance from the deposition zone 105. This space that
separates the reaction zone 126 from the deposition zone 105 is
provided partially by the perpendicular orientation of the
induction coupled plasma torch 102 to the deposition surface 106 of
the deposition target 104 and also partially by the distance
between the induction coupled plasma torch 102 and the deposition
surface 106 of the deposition target 104. This separation provides
for improved deposition efficiency by and through the larger area
of the deposition surface 106 of the deposition target 104. This
separation further allows for increased plasma temperatures in the
reaction zone 126, while maintaining lower temperatures at the
deposition zone 105. This higher plasma temperature in the reaction
zone provides for improved conversion efficiency of the endothermic
chemical reactions in the reaction zone 126. In addition, the lower
temperature in the deposition zone 105 ensures that the desired
characteristics and quality of the silicon is deposited on the
deposition surface 106 of the target substrate 104.
The induction coupled plasma torch 102 further includes a copper
induction coil 114 that is located around the upper portion of the
outer quartz tube 108. The coil 114 comprises a plurality of
windings 116 having a diameter of approximately in the range of
from about 56 mm to about 96 mm. Preferably, the plurality of
windings 116 has a diameter of about 82 mm. Typically, the
plurality of windings 116 are spaced apart from each other by a
sufficient distance to provide for operation of the induction
coupled plasma torch 102. Preferably, the plurality of windings 116
are spaced apart from each other by about 6 mm. In addition, a gap
between the outer quartz tube 108 and the coil 114 can be in a
range of from about 2 mm to about 10 mm.
The induction coupled plasma torch 102 further includes a pair of
injection ports 118 that are connected to a precursor source
chemical line (not shown) carrying the precursor source chemicals
to the induction coupled plasma torch 102. With the use of the
inner quartz tube 110, the plasma source gas will have a swirl flow
pattern. The source chemicals for deposition of semiconductor thin
film material such as silicon will be injected through the
injection ports 118, which are preferably located near the lower
side of the induction coupled plasma torch 102 and aimed toward the
V=0 position for the same reason as disclosed in U.S. Pat. No.
6,253,580 issued to Gouskov et al. and U.S. Pat. No. 6,536,240
issued to Gouskov et al, both of which are incorporated herein by
reference. In one embodiment, the injection ports 118 are connected
to the induction coupled plasma torch 102. In another embodiment,
the injection ports 118 are not connected to the induction coupled
plasma torch 102, but are connected to another structural element
of the present invention as herein described. In one embodiment,
the induction coupled plasma torch 102 is an inductively coupled
plasma torch. The injection ports 118 comprise quartz tubing
preferably having a diameter in the range of from about 3 mm to
about 10 mm, more preferably of about 5 mm, although tubing
diameters in other sizes may be used with the induction coupled
plasma torch 102. In this embodiment, a pair of injection ports 118
are positioned diametrically across from each other. In another
embodiment of the present invention, three or more ports,
symmetrically arranged, may be utilized.
Further, induction coupled plasma torch 102 includes a pair of
plasma gas inlets 120 that are connected to a plasma gas supply
line (not shown) carrying plasma gases to the induction coupled
plasma torch 102. The plasma gas inlets 120 enter the induction
coupled plasma torch 102 at substantially the same height.
Preferably, these plasma gas inlets 120 comprise stainless steel
tubing having a diameter of 5 mm, although a range of diameters may
suffice for this purpose.
The induction coupled plasma torch 102 is also provided with a
coolant inlet 122 and coolant outlet 124. During use, a coolant,
such as water, passes through the coolant inlet 122, circulates
within the stainless steel chamber 112, and exits through the
coolant outlet 124. The coolant inlet 122 and coolant outlet 124
are preferably formed from stainless steel and have a diameter of 5
mm, for example.
The plasma gas inlets 120, the coolant inlet 122 and the coolant
outlet 124 are all preferably formed in a stainless steel chamber
112. The chamber 112 is preferably a stainless steel square block
80 mm on a side, and having a height of approximately 40 mm, for
example. Preferably, the chamber 112 is mounted onto the support
stand (not shown).
A high frequency generator (not shown) is electrically connected to
the coil 114, powering it with a variable power output up to 60 kW
at a frequency of 5.28+/-0.13 MHz. In an embodiment, the generator
is Model No. IG 60/5000, available from Fritz Huettinger Electronic
GmbH of Germany. Preferably, this generator is driven with a 50 Hz,
3-phase, 380 V power supply to energize the induction coupled
plasma torch 102.
FIG. 2 shows another embodiment 200 of a deposition apparatus that
consists of a set of induction coupled plasma torches 102 located
within a deposition chamber 202. By using a plurality of induction
coupled plasma torches 102, located within the deposition chamber
202, all oriented substantially perpendicular to the deposition
surface 106 of the target substrate 104, the deposition apparatus
200 covers a wider deposition width or area. As described above,
the target substrate 104 is moved upward away from the induction
coupled plasma torch 102 and is also rotated about its axis 107 by
the support 103. The target substrate 104 is shown extending almost
to the perimeter of the deposition chamber 202. In addition to a
high deposition rate, the deposition apparatus 200 provides uniform
deposition thickness. In this embodiment, the deposition apparatus
200 consists of five induction coupled plasma torches 102 each
having a diameter of preferably 70 mm. Four of the five induction
coupled plasma torches 102 are spaced equally apart from each other
around the perimeter of the deposition chamber 202. In this
embodiment, the four induction coupled plasma torches 102 located
around the perimeter of the deposition chamber 202 are separated by
90.degree. from each other. In this embodiment, the fifth induction
coupled plasma torch 102 is located in the center of the deposition
chamber 202.
Other arrangements and sizes of induction coupled plasma torches
102 may be used in a deposition chamber to provide for a desired
deposition width or area for a particular application. In this
embodiment, the use of five induction coupled plasma torches 102
will produce a deposition area of approximately 300 mm. Preferably,
the target substrate 104 will rotate around its axis 107 while also
being moved upwards or away from the induction coupled plasma torch
102 to maintain a fixed or constant distance between the target
substrate 104 and the induction coupled plasma torches 102.
Referring to FIG. 3, a side view of the deposition apparatus 200 is
shown. The deposition chamber 202 includes exhaust ports 302 that
are located at the top end of the deposition chamber 202.
Preferably, the exhaust ports 302 are located above the lower end
or deposition surface 106 of the deposition target 104. An exhaust
system (not shown) will remove all gases and any un-deposited
silicon particles from the chemical reactions. Preferably, the
exhaust system controls or maintains a fixed partial pressure
inside the deposition chamber 202 to ensure an optimum deposition
condition. The control of the partial pressure within the
deposition chamber 202 may further include providing a negative
pressure, such as a vacuum. In another embodiment, the partial
pressure may be controlled at or near atmospheric pressure. Any
number of exhaust ports 302 may be employed as desired for a
specific application. Preferably, the deposition chamber 202 is
made of an explosive proof material and RF shield material for
preventing escape of RF energy from the deposition chamber 202 and
for isolating the environmental influences upon the deposition
chamber 202.
By using a plurality of induction coupled plasma torches 102,
located within the deposition chamber 202, all oriented
substantially perpendicular to the deposition surface 106 of the
target substrate 104, the deposition apparatus 200 covers a wider
deposition width or area. The target substrate 104 is shown
extending almost to the perimeter of the deposition chamber 202. In
addition to a high deposition rate, the deposition apparatus 200
provides uniform deposition thickness. In this embodiment, the
deposition apparatus 200 consists of five induction coupled plasma
torches 102 each having a diameter of preferably 70 mm. Four of the
five induction coupled plasma torches 102 are spaced equally apart
from each other around the perimeter of the deposition chamber 202.
In this embodiment, the fifth induction plasma torch is located in
the center of the deposition chamber 202.
FIG. 4 shows another embodiment 400 of a deposition apparatus that
consists of an induction coupled plasma torch 102 in between two
induction coupled plasma torches 402 that are slightly tilted, the
induction coupled plasma torches 102 and 402 are located within a
deposition chamber 202. Induction coupled plasma torches 402 are
constructed and function similarly to induction coupled plasma
torch 102, but they are slightly titled from a horizontal plane by
.theta. degrees within the deposition chamber 202. The degrees of
tilt of the induction coupled plasma torches 402 is preferably from
about 15 degrees to about 45 degrees. Preferably, the induction
coupled plasma torches 402 are tilted approximately 15 degrees from
the horizontal plane. The tilted induction coupled plasma torches
402 provide improved deposition of polycrystalline silicon on the
deposition surface 106 of the target substrate 104 with good
uniformity. It is noted that if the angle of tilt for the induction
coupled plasma torch 402 is too great, then the deposition rate or
collection efficiency decreases and the deposition becomes less
uniform. Further, the degree of tilt for each induction coupled
plasma torch 402 may be different.
From FIG. 4, it is observed that the diameter "L'" of the induction
coupled plasma torch 402 can be deduced from the formula: L'=L/Cos
.theta.>L. Thus, the deposited diameter L' from the induction
coupled plasma torch 402 is larger than the diameter of the
induction coupled plasma torch 102.
As described above, the plasma source gas will have a swirl flow
pattern. This is caused by the plasma source gas being injected
through the plasma gas inlets 120 that feeds the plasma source gas
between the outer quartz tube 108 and the inner quartz tube 110.
The induction coupled plasma torches 102 and 402 preferably uses
the inert plasma source gas to form the plasma where the reaction
takes place between the precursor gas source and the induction
coupled plasma torches 102 and 402 for depositing the reaction
product on the target substrate 104. The plasma source gas will be
an inert gas that preferably has a (i) low activation energy, and
(ii) is chemically inert such that no oxides or nitrides will be
formed. Preferably, the plasma source gas may be selected from the
group including helium argon, hydrogen, or a mixture of them.
The reaction product is produced by the reaction of the precursor
gas sources in the presence of the induction coupled plasma torches
102 and 402. The precursor gas source may include or be additional
forms of matter such as gases, vapors, aerosols, small particles,
nanoparticles, or powders. In addition, a p-type or n-type dopant
material may also be injected with the precursor gas source
simultaneously to form the desired p-type or n-type semiconductor.
Some examples of dopant materials include boron, phosphorous, and
the like.
In addition to the aforementioned aspects and embodiments of the
present plasma deposition apparatuses 100, 200, and 400, the
present invention further includes methods for manufacturing these
polycrystalline silicon substitutes or ingots. One preferred method
includes a chloride based system that utilizes the plasma flame or
energy to reduce trichlorosilane (SiHCl.sub.3) by hydrogen
(H.sub.2) to form silicon. It can also reduce silicon tetrachloride
(SiCl.sub.4) with hydrogen by the plasma flame energy to make
silicon.
FIG. 5 illustrates a flow diagram of an embodiment 500 of one such
process. In step 502, the induction coupled plasma torch or torches
102 and 402 are initiated. This step can include initiating the
flow of the plasma gas supply to the plasma gas inlets 120 and then
plasma ignition by supplying electricity to the induction coil 114.
This step includes igniting and stabilizing the plasma flame of the
induction coupled plasma torch or torches 102 and 402. In addition,
step 502 may also include selecting the precursor gas source to be
used to produce the desired reaction product during deposition on
the target substrate 104.
In step 504, the deposition apparatuses 100, 200, and 400 inject
the precursor gas source through the injection ports 118 to the
plasma flame of the induction plasma torch or torches 102 and 402.
As discussed above, preferably the precursor gas source is selected
from SiHCl.sub.3 plus H.sub.2, or SiCl.sub.4 plus H.sub.2. In step
506, the plasma flame or the induction plasma torch or torches 102
is stabilized and the reaction temperature of the induction plasma
torch or torches 102 and 402 in the reaction zone 126 is adjusted
to optimize the formation of polycrystalline silicon.
As described above, the gases that are not deposited on the
deposition surface 106 of the target substrate 104 are collected
through the exhaust system and recycled for additional use. In one
aspect of the present method for making polycrystalline silicon,
the SiHCl.sub.3 and SiCl.sub.4 can be made from metallurgical grade
silicon (MGS) or Silica. They will react with Hydrogen Chloride
(HCl) that is collected and separated from the exhaust gas stream
of the present process for making polycrystalline silicon. In
addition, it is always possible to add fresh Chlorine (Cl.sub.2) or
HCl, if sufficient quantities do not exist from the exhaust stream.
After purification by distillation, reaction products can be used
as precursor source gas chemicals for making silicon.
In addition to HCl in the exhaust stream, there are Ar, H.sub.2,
dichlorosilane (SiH.sub.2Cl.sub.2), and un-reacted SiHCl.sub.3 and
SiCl.sub.4 plus the un-deposited silicon particles may also exist.
The un-deposited silicon particles can be separated out by using a
bag filter. Further, using a cold trip, chlorosilanes can be easily
separated and reused as precursor source gas chemicals. The gases
such as Ar and H.sub.2 can also be recycled from the exhaust system
and can be used for plasma source gas or precursor source gas.
In step 508, the pressure within the deposition chamber 202 is
controlled and maintained by the exhaust system. In addition, other
means may be employed to maintain the pressure within the
deposition chamber 202. In step 510, the temperature of the
deposition surface 106 of the target substrate 104 is controlled
and maintained to optimize the deposition of the silicon onto the
deposition surface 106. In step 512, the growth of the deposition
surface 106 of the target substrate 104 is monitored. As the
deposition surface 106 grows, the support 103 moves the target
substrate 104 away from the induction plasma torch or torches 102
and 402 to maintain a constant or fixed distance L between the
induction plasma torch or torches 102 and 402 and the deposition
surface 106 of the target substrate 104. In step 514, the support
103 removes the target substrate 104 from the deposition chamber
202 when the desired length or volume of silicon is deposited.
In addition to the above, the silicon particles will be separated
out from the exhaust stream. These particles will be collected,
loaded into a quartz crucible, melted and grow into single crystal
ingots. All the gases whether un-reacted or by-products chemicals
will also be collected and separated by typical industry processes.
Some exemplary raw materials include hydrides, fluorides,
chlorides, bromides, and argon gas.
In another embodiment of the present method for making
polycrystalline silicon, a hydride based system is employed. Silane
does not have high deposition rate as trichlorosilane, but it is
still widely used in the industry, because it is much easier to
purify and also to produce desired high quality silicon. Following
the same processing steps above, Silane (SiH.sub.4) or Disilane
(Si.sub.2H.sub.6) in the gas form can be delivered to the injection
ports 118 as stated in step 504 and in the presence of the plasma
flame or energy they will dissociate into silicon and hydrogen. By
using a higher reaction temperature and removal of hydrogen gas
quickly improved chemical reaction conversion is achieved. In
addition, the un-deposited silicon particles and plasma source gas,
such as Argon, are collected through the exhaust ports 302 for
re-processing and recycling.
In another embodiment of the present methods for making
polycrystalline silicon, a bromine system is employed following the
process steps described above. Both bromine (Br.sub.2) is
chemically less aggressive and also less corrosive than chlorine
(Cl.sub.2). When using Br as a laden gas, a significant equipment
costs can be saved. The laden gas is used as a transporting agent
to bring, convert, and make the dirtly silicon (metallurgical grade
silicon, MGS) into pure and useable solar grade silicon (SoG). It
will react with the MGS to form Silicon Bromide (main product) and
other impurities bromide compounds. After purification, Silicon
Bromide is used for making polycrystalline silicon by plasma
process. During the process, it decomposes the Silicon Bromide into
silicon and bromine. The silicon is deposited and bromine is also
collected and reused again. Because the present induction coupled
plasma torches 102 and 402 have more than enough energy to drive
the reaction in the desirable direction, it will not be a concern
for the reduction reaction of silicon tetrabromide (SiBr.sub.4) by
hydrogen. Preferably, the raw material for this system will be MGS.
At temperatures higher than 360.degree. C., the reaction rate
between Silicon and hydrogen bromide (HBr) or Br.sub.2 can be high
and the reaction product will be mainly SiBr.sub.4. Due to the
differences in boiling temperatures, it is very easy to separate
out the Boron contamination (BBr.sub.3 from SiBr.sub.4). In this
embodiment, the precursor source gas chemicals will be Silicon
tetrabromide and Hydrogen.
In yet another embodiment of the present methods for making
polycrystalline silicon, a reduction of silica soot particles by
carbon is employed. In optical preform production, the solid waste
is the silica soot particles and they usually are sent to a
landfill for disposal. These silica soot particles are very pure
and can be a good source for making Solar Grade Silicon (SoG) by
the carbothermic reduction reaction with carbon. Typically, it uses
an electric arc furnace as a heat source and following the process
steps described above, a powder form of SiO.sub.2 and carbon are
injected through the injection ports 118 into the plasma flames of
the induction coupled plasma torches 102 and 402. These soot
particles from preform manufacturers do not typically contain
transition metal ions and also they do not typically contain boron.
Nevertheless, the soot particles may have trace amount of
phosphorous and some germanium. To eliminate the possible impurity
contamination from the raw materials, small amount of Cl.sub.2 and
moisture can be injected with the precursor gas source. This
embodiment converts the soot particle waste from optical fiber
manufacturing plant into a useful product for producing
polycrystalline silicon, and thus generating efficient and cost
effective solar panels.
In another aspect of the present methods for making polycrystalline
silicon, the target substrates 104 may be removed from the process
at other times than described above to measure the thicknesses,
compositions, and/or performance of the deposition process to
determine whether to adjust any of the process parameters described
above.
Although there has been described what is at present considered to
be the preferred embodiments of the plasma deposition apparatus and
methods for making polycrystalline silicon, it will be understood
that the present plasma deposition apparatus can be embodied in
other specific forms without departing from the spirit or essential
characteristics thereof. For example, additional induction coupled
plasma torches or different combinations of deposition modules,
other than those described herein could be used without departing
from the spirit or essential characteristics of the present plasma
deposition apparatus and methods for making polycrystalline
silicon. The present embodiments are, therefore, to be considered
in all aspects as illustrative and not restrictive. The scope of
the invention is indicated by the appended claims rather than the
foregoing description.
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